direct mass spectrometry of polymers. vii. primary thermal fragmentation processes in polycarbonates

Direct Mass Spectrometry of Polymers. VII.* Primary Thermal Fragmentation Processes in

Pol ycar bonat es

SALVATORE FOTI, MARIO GIUFFRIDA, PIETRO MARAVIGNA, and GIORGIO MONTAUDO, Ist i tuto Dipartimentale di Chimica e

Chimica Industriale, Universitci di Catania, Viale A. Doria 6 , 95125 Catania, I taly

Synopsis

The primary fragmentation mechanisms in the thermal decomposition of several polycarbonates were studied by direct pyrolysis into the mass spectrometer. Our results indicate that ester exchange reactions predominate in the primary thermal fragmentation process of polycarbonates, causing the formation of cyclic oligomers.

INTRODUCTION

The characterization of polymers by direct pyrolysis into the mass spectrometer yields important structural information.14 Typical applications of this method include structural identification of homopolymers, differentiation of isomeric structures, copolymer composition and sequential analysis, identification of oligomers formed in the polymerization reactions, and identification of volatile additives contained in polymer samp1es.l Furthermore, direct pyrolysis into the mass spectrometer (MS) provides unique data in regard to the primary processes of thermal decomposition of polymers.

In previous studies we have investigated several classes of polycondensates by direct MS m e t h ~ d s . ~ - l ~ Here we report a similar study for some polycarbonates.

EXPERIMENTAL

Materials

Basic materials were commercial products appropriately purified before use.

Polycarbonates

Polymerizations were carried out by interfacial polycondensation. A typical procedure (Polymer 111, Table I) is reported here: in the container of a Waring blender, precooled in a refrigerator, were placed 4.56 g (0.02 mol) of isopropylidenediphenol and 2.4 g (0.06 mol) of NaOH dissolved in 24 mL of water. To the

* For Part VI see ref. 12.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 21,1567-1581 (1983) 0 1983 John Wiley & Sons, Inc. CCC 0360-6376/83/061567-15$02.50

1568 FOTI ET AL.

rapidly stirred system was added a solution of 4.52 g (0.021 mol) of butanediol- bischloroformate in 4 mL of CHC13. The mixture was stirred for 10 min and then poured in an excess of water. Chloroform was removed in a rotovapor and the polymer was filtered, washed with water, and dried; then it was purified by dis- solving in dimethylformamide and reprecipitating in water. After standing overnight the polymer was filtered and dried in uacuo a t 5OOC; yield 98%. In the case of resorcinol polycarbonate, phosgene in toluene was used instead of chloroformate in CHC13.

Viscometry

Inherent viscosities [qi,h = In qJc] were measured in a Desreux-Bishoff sus- pended-level viscometer, maintaining the temperature at 30 f 0.01OC. The solvents used are reported in Table I.

Mass Spectrometry

Pyrolyses were carried out in quartz probes with the direct insertion inlet of an electron impact mass spectrometer, LKB-9000 S, according to a technique described elsewherel; the heating rate was about 10°C/min.

Thermogravimetry

Thermal analyses were obtained with a Perkin-Elmer TGS/2 in a Na atmo- sphere (60 mL/min) and 1O0C/min heating rate. Polymer decomposition temperatures (PDTs) were taken on their TGA inflection temperatures.

TABLE I Structure, Thermal Stability, and Viscosity of the Polycarbonates Investigated

Polymer PDT' ("C) vidb

I 470 O.4Sc

405 0.35d

J"

0-0-O- - H J d 360 0.24=

345 0.2od

111 WJ IV W - - C O - C + W +

a Polymer decomposition temperature. qinh = ln(a,/c); c = 0.5 g/dL. In tetrachloroethane/phenol40/60. In dimethylformamide. In chloroform.

DIRECT MASS SPECTROMETRY. VII 1569

*

-15 * -15 * -15

RESULTS

The thermal stabilities of the polymers investigated, indicated by the maxima in their differential thermogravimetric curves (polymer decomposition temperature, PDT), are compared in Table I. Among the polycarbonates studied, poly(4,4’-isopropylidenediphenyl carbonate), I, is the more stable compound. The thermal stability decreases noticeably in poly( resorcinol carbonate), 11, and a further decrement is observed in polymers I11 and IV, in which an aliphatic chain is introduced into the polymer backbone.

*

Polymer I

An accurate study of the direct pyrolysis into the MS of poly(4,4’-isopropylidenediphenyl carbonate) (compound I, Table I) has been already reported.14 The formation of a cyclic dimer and trimer was observed and was confirmed by comparison with the spectra of authentic ~amp1es.l~

The electron impact (EI) spectrum of polymer I, recorded at the reduced electron energy of 16 eV to minimize EI fragmentation, is shown in Figure 1. The spectrum was obtained at a sample temperature of 290°C. In this low-electron-energy spectrum the molecular ion of the cyclic dimer (mlz 508) is the base peak. The cyclic trimer gives an intense molecular ion at rn lz 762. The EI fragmentation pathways of these cyclic carbonates are summarized in Scheme 1. In the scheme, transitions substantiated by metastable peaks are indicated by an asterisk. The fragment ions produced are listed in Table 11.

-44 -44 -44 -44

Scheme 1 Electron Impact (EI) fragmentation pathways common to the four polycarbonates studied

As shown in Scheme 1 and Table 11, two types of fragmentation reaction ac- count for EI fragments observed in these compounds. One is constituted by the consecutive loss of carbon dioxide (44), starting from the molecular ions. This path produces the ion at rn lz 718 in the cyclic trimer and the ions at rn lz 464 and 420 in the cyclic dimer (fragments a and b in Scheme 1; see also Table 11). The expulsion of carbon dioxide is a well-known EI rearrangement process for carbonates, and it becomes predominant in low-electron-energy spectra.15 Fur- thermore these transitions are supported by the presence of strong metastable peaks.

The other fragmentation path is the loss of 15 mass units (CH,) from the molecular ion and from fragments of type a and b. The methyl radical elimi- nated is that of the isopropylidene group; this process is characteristic of isopropylidenediphenol. This fragmentation yields the ions at rn lz 747 and 703 in the trimer, and the ions at rn lz 493,449, and 405 in the dimer. These transitions are also confirmed by the corresponding metastable peaks. Finally, a metastable peak indicates that the fragment of mass 389 is generated from the fragment at rn lz 405 by loss of 16 mass units (CH4?).

IU t

60

-

20-

'001

-

I

135

1 Q?,

_,

I 100

2jl 22

8

40

5

I 389

1 " 1

350

40

0

50

0

16

2

1 14

1

4-l

703

710 I 0

d

Fig.

1.

Mas

s sp

ectr

um (

16 e

V) o

f th

e pr

oduc

ts o

f th

erm

al d

egra

datio

n fo

r po

lyca

rbon

ate

I at

290

°C.


20 a

150

>

v) z Y c z

E

- ioa

z 0

Y c 3 -I 0 In a m

50

/ I / 482

190 ,A1 210 230 250 270 290 310 330

Fig. 2. Mass fragmentograms of decomposition products for polycarbonate I.

The mass spectrum of polymer I displays further peaks at n l z 482,467, and 423. These ions are not related by metastable transitions to those of the cyclic dimer and trimer. The ion at mlz 482 can be rationalized as the molecular ion of bis(4,4’-isopropylidenediphenol carbonate) (Table 11). Metastable peaks indicate that the fragments at mlz 467 and 423 arise from the 482-mass ion by loss of CH3 and CH3 + COa, respectively. These fragmentations are consistent with the proposed structure.

In the lower mass range, the spectrum of polymer I shows an intense peak of mass 228 which is indicative of isopropylidenediphenol.

TA

BL

E I

1 T

herm

al O

ligom

ers a

nd R

espe

ctiv

e EI F

ragm

ents

for

the

Dir

ect P

vrol

vsis

-Mas

s SD

ectr

omet

rv o

f Pol

vmer

s I-I

V

Poly

mer

T

herm

al o

ligom

er

M+

a b

C

d e

f g

I I1

111

- -

-

747

703

- n

= 3

76

2 71

8

493

449

405

- -

-

n =

2

508

464

-

-

n =

5

680

636

n =

4

544

500

456

n =

3

408

364

-

-

-

-

-

-

110

n =

2

740

696

652

+ n

= 1

37

0 32

6 28

2 -


2 I I cv I I I I

% % I m m I I 1 I I I I

8 F w m I I I I I I

% B I w e I % o m m m I (D m m

0 8

2 8 m m

2 8 m m I N N -3 I I $ w e % 2

w @4 W

W m % I 2 -3 N

m + II II - E f

Q z b X

2 I

1574 FOTI ET AL.

Characteristic fragmentations of this structure are found at rn lz 213,197,134, and 94. Also, these ions are not related by metastable peaks to those of the cyclic oligomers.

Mass fragmentograms (Fig. 2) of the molecular ions of the cyclic oligomers show that the intensities of these peaks reach a maximum at about 300OC and decrease at higher temperatures. Instead, the intensities of the 228- and 482-mass ions increase continuously with increasing temperature, up to 330°C.

The absence of metastable peaks that relate ions 228 and 482 to those of the cyclic oligomers and the shape of the fragmentograms support the interpretation that these peaks are molecular ions of independent thermal fragments formed in the thermal decomposition process.

These open-chain fragments containing phenolic end groups are very likely formed by further thermal degradation of the cyclic oligomers, in a consecutive reaction.

Polymer I1

The spectrum of polymer 11, obtained at low electron energy (16 sV) and sample temperature of 270°C (Fig. 3), indicates that this polymer gives by thermal decomposition cyclic trimer, tetramer, and pentamer. The cyclic tetramer is the compound formed preferentially and its molecular ion is the base peak of the spectrum (mlz 544). The cyclic trimer and pentamer give less intense molecular ions at rnlz 408 and 680, respectively. These cyclic oligomers produce fragment ions essentially by the characteristic consecutive loss of carbon dioxide (Scheme I, fragments 3, b, c , d; Table 11).

Most of these fragmentation pathways are confirmed by metastable peaks. Of course, the loss of 15 mass units is not observed in this polymer.

The consecutive loss of four carbon dioxide molecules from the cyclic tetramer yields the fragment a t rn lz 368 that would correspond to a cyclic ether.

The stability to EI of this structure accounts for the high relative intensity of the ion at rn lz 368. This fragment probably gives the ions at rn lz 276,184, and 92 by subsequent loss of 92 mass units (CsH40). Metastable peaks for these fragmentation are not found.

The spectrum of polymer I1 also gives evidence for the formation of open-chain oligomers containing phenolic end groups. The small peak at rn lz 382 corresponds to the molecular ion of resorcinol-bis(resorcino1 carbonate) (Table 11). Bis(resorcino1 carbonate) displays the molecular ion at mlz 246, and this ion gives a fragment ion at mlz 202 (M - 44) by a process supported by a metastable peak (Table 11). Resorcinol itself gives the molecular ion at rnlz 110.

The mass fragmentograms of the molecular ions of the cyclic and open-chain thermal fragments (Fig. 4) show close analogies with that described for polymer

60

20

- 9

2

246

276

184

20

2

1%

100-

60

- s Y

544

40

8

w

cn

4

cn

L 5

00

45

6 1

1

I

68

0

636

- L

I 1

I I

*

1576 FOTI ET AL.

544 544

I 1 I I r I 1 I 170 140 210 230 250 270 290 310 330 OC

382

170 B1 190 210 230 250 270 290 310 330 OC

Fig. 4. Mass fragmentograms of decomposition products for polycarbonate. 11.

I and also suggest that for polymer I1 the open-chain oligomers are formed by further thermal decomposition of the cyclic structures.

Polymer I11

The thermal formation of four cyclic oligomers can be inferred from the pyrolysis-mass spectral data of polymer 111, obtained at 16 eV and a t a sample temperature of 240°C (Fig. 5).

In this spectrum the cyclic monomer gives the strongest molecular ion (mlz

I'

ioa 00

20

1%

100 -

00

-

20

-

2

521

oa7

090

48

6

427

53

6

565

024

1 8

52

08

1 1

740

I l

i

I.

i

L L

240%

10

.V

O-C

O-O

+CH

,.)S

O-C

O

148

42

55

13

5 1

I

1

1.

L.

100

20

0

1

I 1

, L

3 3

20

'i' 7

300

1578 FOTI ET AL.

370). Other molecular ions of cyclic thermal fragments are found a t rn lz 740, 624, and 486. These cyclic oligomers produce fragment ions by consecutive loss of carbon dioxide and CH3 (Scheme I, products a , b , c , d; Table 11). All these fragmentations are confirmed by metastable peaks. In this polymer, which decomposes thermally at relatively low temperature, the molecular ions of open-chain oligomers are very low, with the exception of the peak at rn lz 228, which corresponds to isopropylidenediphenol. However, a metastable transition shows that this ion is formed, a t least partially, by EI fragmentation from the ion of mass 326.

Polymer IV

The mass spectrum of polymer IV reveals the formation of six cyclic oligomers (Fig. 6). The spectrum is obtained at 16 eV and at a sample temperature of 270OC. The molecular ions of these oligomers are not very intense, probably because of the instability of these partially aliphatic structures to EI, even at low electron energy. The cyclic dimer gives the most intense molecular ion in the spectrum (mlz 504). Other molecular ions of cyclic oligomers are found at mlz 368 and 388. EI fragments of these structures are formed by the characteristic losses of carbon dioxide, and are often more intense than the parent molecular ion. For the higher-molecular-weight cycles, molecular ions are not large enough to appear in the plotted spectrum. Thus the molecular ion of the cyclic trimer (mlz 756) and its M-CO2 fragment (mlz 712) does not appear, but EI fragments formed by further ejection of carbon dioxide are present a t rn lz 668,624, and 580.

Again, the molecular ion of another cyclic compound which should appear at r n l z 640 is not detectable, but the EI fragments of this cycle, originating from carbon dioxide elimination, are displayed at rn lz 596,552, and 508.

Molecular ions of open-chain oligomers are not detectable in the spectrum, with exception of the 110-mass ion, which corresponds to the molecular ion of hydroquinone. This is the base peak of the spectrum, but it is related by an intense metastable peak to the ion at mass 164, and this indicates that the ion at mlz 110 is principally an EI fragment of higher masses.

THERMAL DEGRADATION MECHANISMS

The mass-spectral data relative to polymers I-IV (Table 11) indicate that intramolecular ester exchange reactions predominate in the primary thermal fragmentation process of polycarbonates, causing the formation of cyclic oligomers. In view of these results, the formation of cyclic oligomers by ester exchange seems to be the primary thermal fragmentation process generally oc- curring for polycarbonates. The data in Table I1 also summarize the type and size of the cyclic compounds formed in the thermal fragmentation of the polymers.

According to the random nature of the cleavage reaction (ester exchange), the size and relative abundance of the cycles produced is in agreement with the conformational requirements imposed by the structure of the repeating unit in each polymer.


b 0

I

N \ E

0 ;? 0 A.

-I 0

I 0

L rc

1580

Therefore, cyclic dimer and trimer predominate in the MS spectrum of bis- phenol polycarbonate I (Fig. l), while in the case of resorcinol polycarbonate I1 (Fig. 3) the cyclic tetramer largely predominates over the trimer and pentamer.

In the case of aliphatic polycarbonates I11 and IV (Figs. 5 and 6, respectively), the relative abundances of the cyclic oligomers formed in the pyrolysis are esti- mated with more difficulty because of the high lability of these compounds to EI (Table 11).

However, in these cases all types of cyclic compounds predictable on the basis of the intramolecular ester-exchange reaction are observed.

Further insight into the mechanism of thermal decomposition of these polycarbonates is given by the volatilization profiles of the fragments originating in the MS. In Figure 2 are reported the fragmenbgrams relative to polymer I. The cyclic dimer and trimer (mlz 508 and 762) are formed first, but at about 300°C their intensity decreases, as two open-chain fragments (mlz 228 and 482, Table 11) are formed. The pattern is typical of consecutive reaction kinetics and can be explained by assuming that the cyclic carbonates, once formed, undergo hydrolytic cleavage to yield open-chain fragments that contain phenol end groups.

The same situation arises in the case of resorcinol polycarbonate 11. The volatilization profiles of the fragments originating from the pyrolysis of polymer I1 are shown in Figure 4. Again, the cyclic trimer, tetramer, and pentamer (mlz 408,544, and 680, respectively) are generated first, but at about 27OOC their intensity decreases, as three open-chain fragments (mlz 110,246, and 382, Table 11) are formed. Open-chain fragments are also found in the case of aliphatic polycarbonates I11 and IV (fragments at mlz 228 and 110, respectively), showing that the hydrolytic cleavage of the cyclic compounds formed in the primary thermal fragmentation process is a feature common to all polycarbonates investigated.

Our data show also that this secondary thermal process (cleavage) is temperature dependent, so that, at euffkiently low temperatures, the pyrolysis of polycarbonates produces only cyclic oligomers. Therefore, we may conclude that polycarbonates undergo a remarkably selective thermal degradation process.

This behavior is not uncommon among polycondensates, as we have previously rep~r ted .~- '~ In fact, in most of the polycondensates studied, it has been found that the primary thermal fragmentation processes is remarkably selective.

Furthermore, some thermal degradation processes appear to be highly characteristic of certain structures or functional groups, and occur generally in all polymers containing these structures.

Financial Support from the Italian Ministry of Public Education is gratefully acknowledged (Grant No. 20120201/81).

References

1. S. Foti and G. Montaudo, in Analysis of Polymer S y s t e m , L. S . Bark and N. S. Allen, Ede.,

2. R. Adams, J. Polym. Sci. Polym. Chem. Ed., 20,119 (1982). 3. R. M. Lum, J. Polym. Sci. Polym. Chem. Ed., 17,206 (1979). 4. Y. Shimizu and B. Munson, J. Polym. Sci. Polym. Chem. Ed., 17,1981 (1979).

Applied Science, London, 1982, p. 103.

1581

5. R. D. Sedgwick, in Deuelopments in Polymer Characterization-1, J. D. Dawkins, Ed., Applied

6. G. Montaudo, M. Przybylski, and H. Ringsdorf,Makromol. Chem., 176,1753,1763 (1975). 7. A. Ballistreri, S. Foti, P. Maravigna, G. Montaudo, and E. Scamporrino, J. Polym. Sci. Polym.

8. A. Ballistreri, S. Foti, P. Maravigna, G. Montaudo, and E. Scamporrino, Makromol. Chem.,

9. S. Foti, P. Maravigna, and G. Montaudo, J. Polym. Sci. Polym. Chem. Ed., 19, 1679

10. S. Caruso, S. Foti, P. Maravigna, and G. Montaudo, J. Polym. Sci. Polym. Chem. Ed., 20,1685

11. S. Foti, A. Liguori, P. Maravigna, and G. Montaudo, Anal. Chem., 54,674 (1982). 12. S. Foti, P. Maravigna, and G. Montaudo, Macromolecules, 15,883 (1982). 13. S. Foti, P. Maravigna, and G. Montaudo, J. Polym. Degr. Stab., 4,287 (1982). 14. R. H. Wiley, Macromolecules, 4,254 (1971). 15. P. Brown and C. Djerassi, J. Am. Chem. SOC., 88,2469 (1966); 89,2711 (1967).

Science, London, 1978, p. 217.

Chem. Ed., 18,1923 (1980).

181,2161 (1980).

(1981).

(1982).

Received August 4,1982 Accepted October 25,1982

direct mass spectrometry of polymers. vii. primary thermal fragmentation processes in polycarbonates

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